Simple string interpolation.

This allows "%(...)" inside a string literal to interpolate the
stringified result of an expression.

It doesn't support custom interpolators or format strings, but we can
consider extending that later.

test/language/interpolation/empty.wren

@@ -0,0 +1,2 @@
+// expect error line 3
+" %() "

test/language/interpolation/interpolation.wren

@@ -0,0 +1,15 @@
+// Full string.
+System.print("%(1 + 2)") // expect: 3
+
+// Multiple in one string.
+System.print("str%(1 + 2)(%(3 + 4)\%%(5 + 6)") // expect: str3(7%11
+
+// Nested.
+System.print("[%("{%("in" + "ner")}")]") // expect: [{inner}]
+
+// Ignore newlines in template.
+System.print("[%(
+
+"template"
+
+)]") // expect: [template]

test/language/interpolation/runtime_error_in_expression.wren

@@ -0,0 +1 @@
+System.print("%(123.badMethod)") // expect runtime error: Num does not implement 'badMethod'.

test/language/interpolation/switch_fiber.wren

@@ -0,0 +1,8 @@
+var fiber = Fiber.new {
+  System.print("in fiber")
+  Fiber.yield("result")
+}
+
+System.print("outer %(fiber.call()) string")
+// expect: in fiber
+// expect: outer result string

test/language/interpolation/unterminated.wren

@@ -0,0 +1,2 @@
+" %(
+// expect error

test/language/interpolation/unterminated_expression.wren

@@ -0,0 +1,2 @@
+// expect error line 2
+" %(123"